In the fourth project year we will continue to study the effects of hypobaric hypoxia upon platelet sequestration at 9600 and 17,500 feet in an effort to further elucidate its role in the pathogenesis of high altitude pulmonary edema. Electroencephalograms will be done at these altitudes during rest and sleep, and correlated with blood gases. Changes in carbohydrate, lipid and protein metabolism will be surveyed by fasting blood levels of glucose, lactate, free fatty acids, insulin, glucagon, growth hormone and thyroid parameters, and tolerance studies obtained on some of these. Exercise metabolism will also be explored using muscle biopsy and some of the above blood measurements. Fluid and electrolyte intake/output will be measured daily, along with clinical appraisals.